Bug 803575 - Enable preemptive tab zombification in low-memory conditions. r=mfinkle

mobile/android/app/mobile.js

@@ -30,8 +30,8 @@ pref("browser.tabs.remote", false);
 
 // If a tab has not been active for this long (seconds), then it may be
 // turned into a zombie tab to preemptively free up memory. -1 disables time-based
-// zombification (low-memory conditions may still require the tab to be zombified).
-pref("browser.tabs.zombieTime", -1);
+// expiration (but low-memory conditions may still require the tab to be zombified).
+pref("browser.tabs.expireTime", 3600);
 
 // From libpref/src/init/all.js, extended to allow a slightly wider zoom range.
 pref("zoom.minPercent", 20);

mobile/android/chrome/content/browser.js

@@ -235,6 +235,7 @@ var BrowserApp = {
     ExternalApps.init();
     MemoryObserver.init();
     Distribution.init();
+    Tabs.init();
 #ifdef MOZ_TELEMETRY_REPORTING
     Telemetry.init();
 #endif
@@ -511,6 +512,7 @@ var BrowserApp = {
     ExternalApps.uninit();
     MemoryObserver.uninit();
     Distribution.uninit();
+    Tabs.uninit();
 #ifdef MOZ_TELEMETRY_REPORTING
     Telemetry.uninit();
 #endif
@@ -670,7 +672,7 @@ var BrowserApp = {
     evt.initUIEvent("TabOpen", true, false, window, null);
     newTab.browser.dispatchEvent(evt);
 
-    Tabs.zombifyLru();
+    Tabs.expireLruTab();
 
     return newTab;
   },
@@ -7709,13 +7711,43 @@ var Tabs = {
   // of tabs. Each tab has a timestamp associated with it that indicates when
   // it was last touched.
 
+  _enableTabExpiration: false,
+
+  init: function() {
+    // on low-memory platforms, always allow tab expiration. on high-mem
+    // platforms, allow it to be turned on once we hit a low-mem situation
+    if (Cc["@mozilla.org/xpcom/memory-service;1"].getService(Ci.nsIMemory).isLowMemoryPlatform()) {
+      this._enableTabExpiration = true;
+    } else {
+      Services.obs.addObserver(this, "memory-pressure", false);
+    }
+  },
+
+  uninit: function() {
+    if (!this._enableTabExpiration) {
+      // if _enableTabExpiration is true then we won't have this
+      // observer registered any more.
+      Services.obs.removeObserver(this, "memory-pressure");
+    }
+  },
+
+  observe: function(aSubject, aTopic, aData) {
+    if (aTopic == "memory-pressure" && aData != "heap-minimize") {
+      this._enableTabExpiration = true;
+      Services.obs.removeObserver(this, "memory-pressure");
+    }
+  },
+
   touch: function(aTab) {
     aTab.lastTouchedAt = Date.now();
   },
 
-  zombifyLru: function() {
-    let zombieTimeMs = Services.prefs.getIntPref("browser.tabs.zombieTime") * 1000;
-    if (zombieTimeMs < 0) {
+  expireLruTab: function() {
+    if (!this._enableTabExpiration) {
+      return false;
+    }
+    let expireTimeMs = Services.prefs.getIntPref("browser.tabs.expireTime") * 1000;
+    if (expireTimeMs < 0) {
       // this behaviour is disabled
       return false;
     }
@@ -7732,9 +7764,9 @@ var Tabs = {
         lruTab = tabs[i];
       }
     }
-    // if the tab was last touched more than browser.tabs.zombieTime seconds ago,
+    // if the tab was last touched more than browser.tabs.expireTime seconds ago,
     // zombify it
-    if (lruTab && (Date.now() - lruTab.lastTouchedAt) > zombieTimeMs) {
+    if (lruTab && (Date.now() - lruTab.lastTouchedAt) > expireTimeMs) {
       MemoryObserver.zombify(lruTab);
       return true;
     }